COLUMBIA, Md. (AP) — The fountain inside the mall was littered with white flowers. The skateboard shop — the scene of the deadly weekend shooting — was boarded up, as if under construction. Outside the mall, a banner read: "Forever in Our Hearts."

As the Mall in Columbia reopened Monday and shoppers and workers tried to get back to normal, there were reminders of the carnage everywhere. Shoppers wiped away tears. People signed memorial books and photographed the exterior of the store where the killings occurred. Politicians shook hands and consoled the grieving.

"I wanted to be here, I suppose, as a symbol of the fact that everyone in Maryland is with the people of Columbia today," said Gov. Martin O'Malley, who stopped for a cup of frozen yogurt at the mall's food court.

In the days since Saturday's shooting, investigators have worked to piece together what drove the 19-year-old gunman to kill two employees of the skateboard shop, but so far answers have been elusive. Police aren't even sure if the gunman, Darion Marcus Aguilar, knew either victim.

"We haven't ruled anything out and we haven't ruled anything in," Howard County Police Chief William McMahon said in an interview.

Aguilar killed himself soon after the shootings. Investigators said he had no past run-ins with police.

Friends describe Aguilar as an avid skateboarder, a quiet and thoughtful teenager who became a vegan after watching a television documentary on how animals were slaughtered, according to Ellis Cropper, a family friend who is serving as a family spokesman.

He graduated high school in the spring, had taken an interest in environmental engineering and planned to start college classes, Cropper said. At the Dunkin' Donuts where he worked, he was recently given the keys and responsibility of opening the shop for business, something his mother celebrated.

"He was always hugging his mother, kissing his mother, just a very compassionate kid," Cropper said.

When Aguilar didn't show up for work early Saturday, his mother filed a missing persons report. Officers went to Aguilar's home several hours after the shooting and saw a journal. Police have said it described his general "unhappiness" but did not provide a motive. However, the contents were enough for an officer to worry about Aguilar.

Courtney, center, a friend of one of the victims in Saturday's shooting at the Mall in Columbia, …

"That's when the officer said, 'Have a seat, let me tell you what's going on in Columbia,'" Cropper said.

Since then, Aguilar's mother has been asking herself, "What did she miss? What did she miss? How could she not have known something?" Cropper said.

Police on Monday denied a request to release copies of 911 calls and incident reports, saying the records are part of an ongoing investigation.

The shooting took place inside Zumiez, a shop on the upper level that sells skateboard gear. The store was covered with white boards Monday, with messages encouraging passers-by to leave condolences for the slain workers at memory books that the mall provided.

Mary Lou Shippe, 71, did errands at the mall, including banking and getting a broken necklace repaired. She said she wanted to "show my support for the merchants, let them know they can't scare us away."

"It's very disturbing something like that would happen here, but today it happens anywhere," she said.

Police say Aguilar's 12-gauge shotgun was legally purchased. Dan Millen, co-owner of United Gun Shop in Rockville, Md., said Aguilar purchased a single-barrel shotgun from him last month along with boxes of shotgun shells. Aguilar said the weapon was intended for home defense and returned to the store later in the month, saying he had tested the shotgun at a firing range and wanted to buy another box of ammunition, Millen recalled Monday.

Aguilar, who had concealed the shotgun in a bag, fired six to nine times in the mall.

"He did not raise any red flags in any way," Millen said, describing Aguilar as nice and pleasant. "We turn people down here a fair amount just if we don't feel comfortable with them. He did not fall into that category at all."

Noah Sturdivant, who had a gym locker next to Aguilar's at their high school, said the two would see each other basically every day. He said Aguilar was into video games and part of "the skate culture" at the school and "not just a quiet loner guy."

"He was never mean to anyone. There's not one instance where I can remember him getting mad and yelling," said Sturdivant, 17. "Cool is a better word than quiet. There wasn't a day when you were like, 'Oh, he's had a bad day.'"

One victim, Brianna Benlolo, a 21-year-old single mother, lived half a mile away from Aguilar in the same College Park neighborhood. The other employee, Tyler Johnson, did not know Aguilar and did not socialize with Benlolo outside of work, a relative has said.

Johnson had been serving as a board member at a 12-step meeting place in Columbia for recovering addicts.

"He was radiant, he was caring, he had straightened out his life — overcoming addiction — and wanted to help others," said Serenity Center volunteer executive director Claudia Friend.